A diffraction based particle characterization instrument works by measuring light scattered from particles. A mathematical description (or model) of the relationship between scattering patterns and particle size distribution is used to infer the particle size distribution. The mathematical description may require, as a calculation parameter, the ratio of refractive index between the particle and the medium in which the particle is suspended. By comparing measured light scattering data to the model a particle size distribution (PSD) may be calculated. The theory that is typically used assumes that the refractive index of the medium is homogenous and static. If the refractive index of the medium is not homogeneous and static during measurement the calculated PSD may include spurious sizes.
Both diffraction based measurements (such as static light scattering) and dynamic light scattering (based on temporal characteristics of scattering) are affected by scattering from inhomogeneities in the diluent medium.
There are several reasons the refractive index of the suspending medium (or dispersant) may be inhomogeneous. The most common is due to thermal variations in the dispersant but other causes can include pressure variations, contaminants, sample dissolution, etc. The scattering pattern caused by these kinds of inhomogeneities may be substantially random in nature, and therefore difficult to remove from measurement data.
JP2015/141025 discloses a device in which isotropic background light is subtracted to allow a scattering measurement to be performed without a filter to remove fluorescent light. U.S. Pat. No. 7,471,393 discloses an instrument for measuring the size distribution of a particle sample by counting and classifying particles into selected size ranges.
Conventionally, the effect of thermal or pressure variations are minimised by waiting for such variations to equalize while holding the sample in a temperature controlled environment. However, conditions can exist where this takes an excessive amount of time or even where settling will not occur. For contaminants or dissolution there is no known general solution.